This invention relates to a developer (for example, a toner) used for developing a latent image in an electrophotographic apparatus. This invention also relates to an image forming apparatus and an image forming method using the developer.
A toner used in the electrophotographic apparatus is required to have a property of preventing an offset. The offset is divided broadly into a hot offset and a cold offset. The cold offset is a phenomena that the toner melts (where the toner contacts a heat roller) and sticks to the surface of the heat roller. The hot offset is a phenomena that the temperature of the molten toner becomes excessively high, and the cohesive power of the toner is reduced, so that the toner sticks to the heat roller.
Recently, it is demanded to make it possible to touch-up a full color image that has been printed by the electrophotographic apparatus. For this purpose, there has been developed a so-called oilless fixing device in which the small amount of oil (or no oil) is coated on the heat roller. In the case where the oilless fixing device is used, it is necessary to increase the elasticity of the toner at high temperature to thereby increase the cohesive power, in order to prevent a printing sheet from being wound around the heat roller. However, if the elasticity of the toner at high temperature is high, the cold offset may easily occur.
In order to solve these problems, it is proposed to use two kinds of waxes having different softening temperatures, as releasing agents contained in the toner. Such a toner is disclosed by, for example, Japanese Laid-Open Patent Publication No. 2003-91094.
In the case where a full color image (i.e., an image of high density) is printed on a printing medium having a large heat capacity such as a thick paper, a large heat energy is absorbed by the printing medium when the image is fixed to the printing medium, and therefore it is necessary to increase the fixing temperature or decrease the feeding speed of the printing medium (i.e., a circumferential speed of the heat roller). However, if the fixing temperature is increased, the temperature difference between respective portions of the heat roller increases, and therefore the temperature may partially be out of the temperature range in which the fixing can be favorably performed. In such a case, the degradation of the fixing quality may occur. Thus, in the conventional technology disclosed by the above described publication, the feeding speed (20 to 150 mm/s) during the printing of the full color image is set to be lower than the feeding speed (80 to 200 mm/s) during the printing of the monochrome image. Accordingly, in the conventional technology, it is not possible to print the full color image at high speed.